In the art of making paper with modern high-speed machines, sheet properties must be continually monitored and controlled to assure sheet quality and to minimize the amount of finished product that is rejected when there is an upset in the manufacturing process. The sheet variables that are most often measured include basis weight, moisture content, and caliper (i.e., thickness) of the sheets at various stages in the manufacturing process. These process variables are typically controlled by, for example, adjusting the feedstock supply rate at the beginning of the process, regulating the amount of steam applied to the paper near the middle of the process, or varying the nip pressure between calendaring rollers at the end of the process. Papermaking devices well known in the art are described, for example, in "Handbook for Pulp & Paper Technologists" 2nd ed., G. A. Smook, 1992, Angus Wilde Publications, Inc., and "Pulp and Paper Manufacture" Vol III (Papermaking and Paperboard Making), R. MacDonald, ed. 1970, McGraw Hill. Sheetmaking systems are further described, for example, in U.S. Pat. Nos. 5,539,634, 5,022,966 4,982,334, 4,786,817, and 4,767,935.
On-line measurements of sheet properties can be made in both the machine direction and in the cross direction. In the sheetmaking art, the term machine direction (MD) refers to the direction that the sheet material travels during the manufacturing process, while the term cross direction (CD) refers to the direction across the width of the sheet which is perpendicular to the machine direction.
Papermaking machines typically have several control stages with numerous, independently-controllable actuators that extend across the width of the sheet at each control stage. For example, a papermaking machine will typically include a headbox having a plurality of slices at the front which allow the stock in the headbox to flow out on the fabric of the web or wire. The papermaking machine might also include a steam box having numerous steam actuators that control the amount of heat applied to several zones across the sheet. Similarly, in a calendaring stage, a segmented calendaring roller can have several actuators for controlling the nip pressure applied between the rollers at various zones across the sheet.
All of the actuators in a stage are operated to maintain a uniform and high quality finished product. Such control might be attempted, for instance, by an operator who periodically monitors sensor readings and then manually adjusts each of the actuators until the desired output readings are produced. Papermaking machines include control systems for automatically adjusting cross-directional actuators using signals sent from scanning sensors.
In making paper, virtually all MD variations can be traced back to high-frequency or low-frequency pulsations in the headbox approach system. CD variations are more complex. Preferably, the cross-direction dry weight profile of the final paper product is flat, that is, the product exhibits no CD variation, however, this is seldom the case. Various factors contribute to the non-uniform CD drainage which ultimately results in fluctuations in the CD profile. These factors include, for example, (i) non-uniform headbox delivery, (ii) clogging of the plastic mesh fabric of the wire, (iii) varying amounts of tension on the wire, and (iv) uneven vacuum distribution.
Cross-directional measurements are typically made with a scanning sensor that periodically traverses back and forth across the width of the sheet material. Current technology in papermaking uses a beta type sensor that scans across the sheet during the manufacturing process to measure basis weight. The objective of scanning across the sheet is to measure the variability of the sheet in both CD and MD. Based on the measurements, corrections to the process are made to make the sheet more uniform. A difficulty with this measurement technique is that while the sensor scans across 30 to 40 feet of the sheet in the CD, 1000 to 2000 feet of paper have passed the sensor in the MD. This means that MD and CD information are mixed together during a scan. Further, the scanning sensor is capable of measuring only a small fraction of the paper produced. The "footprint" area covered by the scanning sensor is typically less than 1% of the total sheet surface. Another disadvantage is that the sheet shrinks as it dries, so corrections must be made to determine which actuator at the headbox will affect the location being measured.
To separate CD information from the mix, it is typical to filter the data from many scans to average out MD variations. With filtering, it takes several minutes to obtain an accurate CD profile. The MD information is usually extracted by using the average of all readings across the sheet, i.e., "scan average." While these methods have proven reliable and accurate over the years, the main disadvantage is that they are slow and only a small fraction of the sheet is actually measured.
As is apparent, there is a need in the art for effective methods of controlling and measuring the dry weight of paper in a papermaking machine especially the CD dry stock weight profile.